Optimistic Active Replication

Replication is a powerful technique for increasing availability of a distributed service. Algorithms for replicating distributed services do however face a dilemma: they should be (1) efficient (low latency), while (2) ensuring consistency of the replicas, which are two contradictory goals. The paper concentrates on active replication, where all the replicas handle the clients' requests. Active replication is usually implemented using the Atomic Broadcast primitive. To be efficient, some Atomic Broadcast algorithms deliberately sacrifice consistency, if inconsistency is likely to occur with a low probability. We present in the paper an algorithm that handles replication efficiently in most scenarios, while preventing inconsistencies. The originality of the algorithm is to take the client-server interaction into account, while traditional solutions consider Atomic Broadcast as a black box.